Here are two links, one to HB & the other for Lorance.
HumminBird 597ci HD DI Combo
http://store.humminbird.com/products...ci_HD_DI_Combo
Lorance Structure Scan:
http://www.lowrance.com/Products/Mar...pecifications/
From what I read on the HB, the max depth for DI is 250 ft. while on the Lorance Structure Scan the max depth is 100 ft. (800kHz) & 300 ft. (455kHz). With teh Structure Scan use still need to purchase a unit to use it with.
I own a HB 798cisi, problem with the "SI" version of HB is that if you want to use that function the transponder must be in the water. This makes for a difficult setup on a kayak. The DI versions are more user friendly in that respect. They also have a smaller "thru hull" transducer that can be gooped or wet mounted to the hull.
Good luck on your search.